1 82 THE VARIATION OF ANIMALS IN NATURE 



of the principle is given in ' The Origin.' As stated in that 

 work the proof consists of four essential parts : 



(a) A demonstration of the efficacy of selection by Man. 



(b) A survey of the circumstances in which Natural Selection 



is assumed to work (numerical increase, struggle for 

 existence, variation, etc.). 



(c) A consideration of the phenomena of adaptation. 



(d) A survey of the facts of ' divergence ' in relation to 



distribution in time and place. 



The occurrence of sundry secondary phenomena of im- 

 portance in the theory (such as correlation and isolation) is 

 also dealt with. 



Throughout the work Darwin does not clearly distinguish 

 between Evolution as an historical process and Natural Selection 

 as the effective agent. A large amount of his data merely 

 serves to prove the occurrence of the former. The following 

 quotation from ' Animals and Plants under Domestication ' 

 (1905, vol. ii, p. 10) serves to illustrate this. ' The principle 

 of Natural Selection may be looked at as a mere hypo- 

 thesis, but rendered in some degree more probable by what 

 we positively know of the variability of organic beings in a 

 state of nature, by what we know of the struggle for exist- 

 ence, and the consequent almost inevitable preservation of 

 favourable variations ; and from the analogical formation 

 of domestic races. Now this hypothesis may be tested— and 

 this seems to me the only fair and legitimate manner of con- 

 sidering the whole question — by trying whether it explains 

 several large and independent classes of facts, such as the 

 geological succession of organic beings, their distribution in 

 past and present times, and their mutual affinities and homo- 

 logies. If the principle of Natural Selection does explain 

 these and other large bodies of facts, it ought to be received. 

 On the ordinary view of each species having been indepen- 

 dently created, we gain no scientific explanation of any one 

 of these facts.' To a modern reader, it cannot but occur 

 that any theory of evolution would explain, say, the facts of 

 homology and geological succession : Natural Selection has 

 no particular advantage in this respect. 



In Darwin's treatment of the subject no proof is adduced 

 that a selective process has ever been detected in nature. 



